Page 97 - C-Language

{
return 5;
}

int main(int argc, char **argv)
{
int foo_val;
foo_val = foo();
return foo_val;
}

Now this code will compile. An alternative situation arises where the source for foo() is in a
separate source file foo.c (and there's a header foo.h to declare foo() that is included in both foo.c
and undefined_reference.c). Then the fix is to link both the object file from foo.c and
undefined_reference.c, or to compile both the source files:

$ gcc -c undefined_reference.c
$ gcc -c foo.c
$ gcc -o working_program undefined_reference.o foo.o
$

Or:

$ gcc -o working_program undefined_reference.c foo.c
$

A more complex case is where libraries are involved, like in the code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

int main(int argc, char **argv)
{
double first;
double second;
double power;

if (argc != 3)
{
fprintf(stderr, "Usage: %s <denom> <nom>\n", argv[0]);
return EXIT_FAILURE;
}

/* Translate user input to numbers, extra error checking
* should be done here. */
first = strtod(argv[1], NULL);
second = strtod(argv[2], NULL);

/* Use function pow() from libm - this will cause a linkage
* error unless this code is compiled against libm! */
power = pow(first, second);

printf("%f to the power of %f = %f\n", first, second, power);

return EXIT_SUCCESS;

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